Missing dope sheet keyframes
You have created some keyframes for your Mix Shader, and if you go in the Dopesheet you can move these keyframes as you want. In the Dopesheet, if you click on the arrow icon button (on the left of the phantom button, you will display only the keyframes related to the object selected on the 3D View, if I enable this option and select the Clouds object I see all the keyframes related to Clouds, and on the left you can see that you can clearly access to its material (Shader Nodetree).